Abstract
One of the greatest inputs of available nitrogen into the biosphere occurs through the biological N2-fixation to ammonium as result of the symbiosis between rhizobia and leguminous plants. These interactions allow increased crop yields on nitrogen-poor soils. Exopolysaccharides (EPS) are key components for the establishment of an effective symbiosis between alfalfa and Ensifer meliloti, as bacteria that lack EPS are unable to infect the host plants. Rhizobium favelukesii LPU83 is an acid-tolerant rhizobia strain capable of nodulating alfalfa but inefficient to fix nitrogen. Aiming to identify the molecular determinants that allow R. favelukesii to infect plants, we studied its EPS biosynthesis. LPU83 produces an EPS I identical to the one present in E. meliloti, but the organization of the genes involved in its synthesis is different. The main gene cluster needed for the synthesis of EPS I in E. meliloti, is split into three different sections in R. favelukesii, which probably arose by a recent event of horizontal gene transfer. A R. favelukesii strain devoided of all the genes needed for the synthesis of EPS I is still able to infect and nodulate alfalfa, suggesting that attention should be directed to other molecules involved in the development of the symbiosis.
Highlights
Alfalfa (Medicago sativa), is a widely cultivated legume used worldwide for feeding cattle mainly due to its high nutritional value, and because its cultivation does not cause a high soil erosion (Li et al, 2011; Li and Brummer, 2012)
As R. favelukesii LPU83 is able to infect alfalfa with some particular features, the orthologs to the genes known to be involved in polysaccharides synthesis were searched for using BLAST
The particular genetic organization that we found for the EPS I gene cluster raises some questions: which is the evolutionary relationship of the genes present in the plasmids? Are both genetic clusters essential for EPS synthesis? Is the EPS produced by LPU83 similar to the one produced by E. meliloti? how relevant is the EPS of LPU83 for the infection of alfalfa?
Summary
Alfalfa (Medicago sativa), is a widely cultivated legume used worldwide for feeding cattle mainly due to its high nutritional value, and because its cultivation does not cause a high soil erosion (Li et al, 2011; Li and Brummer, 2012). This legume establishes a highly specific symbiosis with rhizobia, in which the model symbiont is Ensifer (Sinorhizobium) meliloti. The plant secretes a set of different flavonoids for which rhizobia have a specific receptor, the NodD
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